Light curing restorations are commonly used in dental applications. As part of a typical dental procedure, a composite is dispensed in a tooth cavity, and the composite is hardened or cured with a dental curing light. In conventional systems, halogen or LED curing lamps are used to flood an entire tooth with light so as to cure an entire area of the composite simultaneously. Such flooding of light indiscriminately illuminates the tooth with unfocused light, which makes it challenging to control shrinkage behavior of the composite.
The curing light activates polymerization of the composite as a function of light intensity throughout the cavity. As the composite polymerizes, it naturally shrinks due to formation of covalent bonds and reduction in free volume. When the entire composite starts to polymerize at once, the composite material is trapped in an energetically less stable state in which it does not have enough time and mobility to relax. On the other hand, the composite is bonded or adhered to cavity walls in order to secure the restoration in place and seal a margin or interface. These two phenomena—i.e., total shrinkage of the composite and adhesion to the cavity walls—result in strain, which consequently induces stress on the cavity walls. Such stress concentration at the interfaces between the composite and the cavity walls, referred to as the margins, may result in immediate or delayed debonding, which may result in a number of clinical issues, such as secondary caries. In general, debonding may reduce lifetime and effectiveness of the restoration. Such stress at the interfaces may also lead to post-operative pain and/or sensitivity for the patient.
Current methods of dealing with the problem of uncontrolled stress on the cavity walls may include using a layer filling technique, applying liners in the cavity base, employing light soft-start illumination strategies (e.g., ramp cure), and/or using flowable or low shrinkage composites. Although composites with reduced shrinkage have been developed recently, overall shrinkage and the shrinkage stress for dental materials are still not ideal.